The present invention relates generally to a burn-out furnace and more specifically to a configuration for heating elements contained in the burn-out furnace.
In the manufacture of dental restorations, the dental practitioner prepares the tooth to be restored by grinding the subject tooth or teeth down to form one or more tooth preparations to which the prosthetic device is to be attached. An impression of the tooth preparation is taken in an elastic material and the impression is used to produce a model and dies. At this point, various techniques may be used to fabricate the dental restoration. One such technique involves the lost wax process whereby a pattern of the lost tooth structure or the desired dental prosthesis is constructed in wax on the die. The wax is enveloped by a material called an investment, which is typically a mixture of water, silica, and a binder. After the investment slurry has set, the wax is burned out of the mold in a burn-out furnace. The burn-out furnace serves to burn-out the wax pattern which is used to make the mold shape. It also cures the mold material, a ceramic shell, insuring the mold is perfectly dry prior to casting. It is very important that all the wax has been removed and the mold is dry prior to casting or there will likely be an explosion when liquid metal is poured into the mold.
Current burn-out furnaces may have heating elements disposed vertically in the muffle of the furnace. Due to the vertical positioning of the wires, over time and usage, the wires begin to gravitate and sag to the bottom of the furnace, and eventually collapse into a mass at the bottom of the furnace. This results in uneven heating, creating a heat sink in the lower section of the furnace and a cool section in the upper section of the furnace.
In addition to the vertical arrangement of the heating elements, many of the current burn-out furnaces have heating elements arranged in a horizontally S-type mounting pattern whereby the wires are wound very tightly in order to fit a large volume of wire into a small area. The close proximity of the coil spacing to one another results in inefficient heating, since the wires tend to become very hot internally and are too close to one another to dissipate heat effectively. The heat from the elements should be dissipated into the muffle, but instead, it becomes trapped between the elements. The heating elements may reach their thermal limit prior to radiating enough heat to heat the muffle or to maintain the muffle at the desired temperature, thereby reducing the utility of the furnace.
It is desirable to provide a burn-out furnace having heating elements disposed in such a way as to reduce sagging of the elements. It is preferable that the burn-out furnace be provided with heating elements designed to provide effective and efficient heating to the muffle.
These and other objects and advantages are accomplished by the burn-out furnace of the present invention having a series or plurality of radiating tubes or sleeves aligned on top of one another against at least one wall of the burn-out furnace. At least one heating element is disposed in the radiating sleeves. The heating element is preferably a single piece of wire which is coiled or wound along its entire length and curved or bent after it exits each radiating sleeve, preferably in the form of a tube, to continue into the next tube, and xe2x80x9czig-zagxe2x80x9d through the plurality of tubes. The heating element is stretched to a point whereby during operation of the furnace, when the heating element is heated, there is little or no creeping of the wire occurring. Moreover, the heating element contains very tight turns between the sleeves as the direction of the element reverses and it continues the zig-zag configuration through the remainder of the plurality of radiating sleeves. The burn-out furnace is useful in the manufacture of dental restorations, heat treatment of metal, and general laboratory uses.